The purpose of this article is to explain the procedure used by STAAD in finding the most optimum section for a steel member - a task which is referred to as Member Selection and is initiated using the instruction SELECT ALL or SELECT MEMBER list.
The following are the steps.
1. Start with the first member on the list.
2. Gather the member forces from the most recent analysis for all the load cases shortlisted through the LOAD LIST command. This dataset is known as the member forces dataset.
3. Is deflection a criteria that needs to be used during selection for that member? If the answer is Yes, go to step 7. Else, go to step 4.
4. Start with the least weight section in the section database corresponding to that section type. By section type, we mean wide flange, channel, HSS, etc.
5. Does this section conform to the restrictions imposed by parameters such as PROFILE, DMAX, DMIN, etc.? If the answer is Yes, go to the next step. If the answer is No, pick the next section from the database and repeat this step.
6. Go to step 9.
7. Meeting the deflection criteria : Find the ratio A/B where
A = value of the DFF parameter. B = ratio of length to deflection (denoted by the term lowercase "dff")
If A/B is greater than 1, go to the section database and find the smallest section whose IZ is equal to the IZ of the currently assigned section for that member multiplied by A/B.
8. If this section conforms to the restrictions imposed by parameters such as PROFILE, DMAX, DMIN, etc., go to the next step. Else, find the next larger section (in terms of IZ) and repeat this step.
9. For the member force dataset described in step 2, using the properties of the section chosen from the database, perform the various strength checks and store the highest utilization ratio from all those checks. Call this value HIGHEST_RATIO.
10. If HIGHEST_RATIO is larger than the value specified by the user for the RATIO parameter for that member, go back to the section database and pick the next larger section, making sure that it meets the criteria described in step 5 and, the "IZ" criteria described in step 7 if deflection too is to be checked.
11. If HIGHEST_RATIO is smaller than the value specified by the user for the RATIO parameter for that member, the section is deemed to have passed, and this is reported as the optimum one for that member.